Tracing Multiphase Structure in the Circumgalactic Medium: Insights from Magnetohydrodynamic Turbulence Simulations

Abstract

The circumgalactic medium (CGM) is the diffuse gas surrounding a galaxy's halo, and it plays a vital role in the galactic baryon cycle. However, its mass distribution across the virial phase and the cooler, denser atomic phase, remains uncertain. To investigate this, we perform high-resolution magnetohydrodynamic simulations of 0.125--8 kpc-scale representative patches of the CGM, with parameters informed by quasar absorption line observations. Our simulations resolve the cooling length (the minimum across all temperatures of $c_s t_{\rm cool}$, where $c_s$ is the sound speed and $t_{\rm cool}$ is the cooling time in isobaric conditions), allowing us to track the evolution of cold gas more accurately. We find that low-density CGM gas ($3\times10^{-4}$ cm$^{-3}$) cannot sustain cold gas below $10^4$ K for long, due to a large value of the ratio between the cooling to mixing time ($t_{\rm cool}/t_{\rm mix}$). In contrast, higher-density environments ($3\times10^{-3}~{\rm cm}^{-3}$) reach a turbulent multiphase steady state, with up to $50\%$ of the mass in the cold phase, occupying only about $1\%$ of the volume. To connect with large-volume cosmological simulations and small ${\rm pc}$-scale idealized simulations, we explore different box sizes (0.125--8 kpc) and identify a key scaling relation: simulations with similar $t_{\rm cool}/t_{\rm mix}$ exhibit comparable cold gas mass fractions and lifetimes. Importantly, we find that simply sub-sampling (reducing box-size) a small region from a large-volume simulation while maintaining a constant turbulent energy density injection rate from larger to smaller scales artificially shortens $t_\mathrm{mix}$, leading to inaccurate predictions for cold gas survival. This means that cold gas at small $\lesssim 10$ kpc scales arises in relatively dense, quiescent regions of the CGM rather than the turbulent ones undergoing cascade from large scales.

Figures (13)

• Figure 1: Projections of density (Col 1, vol-weighted), temperature (Col 2, mass-weighted), Cooling rate (Col 3, vol-weighted), and plasma beta (Col 4, mass-weighted) along the $x$-direction for the Fiducial and LDens sets of runs. The streamlines on Columns 2 and 4 depict the mass-weighted projections of the velocity and magnetic field, respectively. Cold, dense gas exists in all runs except the LDensHydro run, where it has mostly evaporated by $t=80~\mathrm{Myr}$. The FidHydro run shows a lot of small, shattered cold clouds that are absent in the FidMHD run. In the MHD runs, we find the cold clouds are filamentary and have a larger fraction of magnetic pressure.
• Figure 2: Similar to \ref{['fig:projection']}, except here we show the slices of density (Col 1), temperature (Col 2), Cooling rate (Col 3), and plasma beta (Col 4) along the $x$-direction for the Fiducial and LDens sets of runs at $t=80~\mathrm{Myr}$. The streamlines on Columns 2 and 4 depict the velocity and magnetic field, respectively. We find more gas at intermediate temperatures/densities for the FidHydro run, compared to the FidMHD run, due to suppressed mixing in the MHD run. An animated version of this figure is available https://www.youtube.com/shorts/PAFFfJaBDCw.
• Figure 3: Time evolution of different statistical properties of the gas for the Fiducial and LDens hydro and MHD runs. Left col: The velocity dispersion is smaller for the MHD runs compared to the hydro runs. For the MHD runs, the plasma beta is a few $100$ in steady state, and both simulations are sub-Alfvenic. Right col: The Fiducial set of runs reach a steady state with roughly half of their mass in the cold phase ($T<10^{4.2}~\mathrm{K}$), which occupies $\lesssim1\%$ of the volume. For the LDens set of runs, $<1\%$ of the mass converts into the cold phase, and it quickly mixes up with the hot phase due to its longer cooling time. The Area covering fraction of gas at $T<10^5~\mathrm{K}$, with column density$>10^{18} \mathrm{cm}^{-2}$, is roughly $80\%$ for the Fiducial set, whereas it reaches a maximum of $40\%$ for the LDens set before dropping to $0$.
• Figure 4: Density-temperature phase diagram (Row 1) for our Fiducial and LDens hydro and MHD runs; also shown are $t_\mathrm{cool}/t_\mathrm{mix}$ and $c_st_\mathrm{cool}/\Delta x$ in Rows 2 and 3, respectively. The contours on Rows 2 and 3 denote the $90\%$ and $99.9\%$ percentile of the density-temperature PDF. The cold and hot gas are mostly isobaric. For LDens set of runs, $t_\mathrm{cool}>t_\mathrm{mix}$ for all gas, whereas for the Fiducial set, $t_\mathrm{cool}\lesssim t_\mathrm{mix}$ for $T\lesssim10^5~\mathrm{K}$. For gas at $T>10^4~\mathrm{K}$, we resolve $c_st_\mathrm{cool}$ at all temperatures and densities by at least 30 cells for all our simulations.
• Figure 5: Density and compensated velocity spectra at different times for our LDens hydro and MHD runs. The density power spectrum is much flatter than Komlogorov (K41), and with time evolution it reduces in amplitude and becomes steeper for both hydro and MHD runs. The velocity power spectrum is slightly steeper than the K41 scaling for hydro runs, whereas for the MHD run it is slightly flatter than $k^{-4/3}$ scaling reported in recent multiphase turbulence studies.